The immune system has an elaborated surveillance function to distinguish normal cells from abnormal cells in the body, and remove only the abnormal cells. Once the surveillance function collapses, however, abnormal cells produced by mutation and the like cannot be removed but are allowed to proliferate in the body. A mass of these proliferated abnormal cells is a tumor, i.e., cancer.
Cancer is mainly treated by a method of surgically removing cancer or a method including use of anti-cancer agents. However, these treatment methods place a physical burden caused by removal surgery or side effects of anti-cancer agents, as well as a mental burden due to operative scars.
In such background, a treatment method using an immunotherapy in combination is drawing attention. In the immunotherapy, the number of immunocytes of patients is increased, and let the activated immunocytes attack cancer cells. If the immunity therapy succeeds in reducing the size of the cancer, the physical burden caused by the removal surgery thereafter becomes small. In addition, since the operative scar is small, the mental burden is drastically reduced.
Natural killer (NK) T cells are immunocytes belonging to a novel lymphocyte lineage showing characteristics different from those of other lymphocyte lineage cells (T, B, NK cells) which has been known ever. Since cytotoxic perforin granules are present in NKT cells, they are analogous to NK cells (non-patent document 1). However, since NKT cells express not only NK cell marker but also T cell receptor (TCR), they form a definitively different from the other lymphocyte lineage cells, new cell (non-patent document 2). NKT cells can produce both Th-1 type cytokine (mainly interferon (IFN)-γ) produced by helper T (Th)-1 cell that promotes immunostimulatory action and Th-2 type cytokine (mainly interleukin (IL)-4) produced by Th-2 cell that promotes immunosuppressive action (non-patent document 3), which suggests a possibility of controlling the balance of immune system (non-patent document 4). Therefore, by an immunotherapy of controlling the function of NKT cells, disrupted balance of the immune system is controlled and the surveillance function is enhanced, whereby cancer can be treated.
The most noticeable characteristic of NKT cells is that the α chain of TCR expressed by NKT cells is common to all members of one species. In other words, this means that all NKT cells of the living organisms belonging to the same species are activated by the same substance. This α chain is Vα24 in human and Vα14 in mouse, and they show extremely high homology between the two species. In addition, only very limited kinds of β chain are known to form a pair with the α chain. For this reason, this TCR is also called a “non-variable TCR”.
There are various kinds of glycosphingolipids which are known to be present in the body. In glycosphingolipids in the body, various sugars generally form a β-bond with ceramide. While the existent amount thereof varies depending on the organ, they are present in the cellular membrane of various organs (non-patent document 5).
In the meantime, a report is known that glycosphingolipids wherein sugar forms an α-bond with ceramide has a strong immunostimulatory action and an antitumor activity. α-Galactosylceramide represented by Agelasphins is a glycolipid isolated from an extract of Agelas mauritianus, one kind of sponge, and has been reported to strongly activate NKT cells (non-patent document 6). α-Galactosylceramide is a sphingoglycolipid wherein galactose is bound by α-configuration to a ceramide formed by acylation of a sphingosine base by a long chain fatty acid.
After intake by antigen presenting cell (APC), which is represented by dendritic cell (DC) and the like, α-galactosylceramide is presented on the cellular membrane by a CD1d protein similar to major histocompatible complex (MHC) class I molecule. NKT cells are activated by recognition using TCR of the thus-presented complex of CD1d protein and α-galactosylceramide, which triggers various immune reactions.
Various analogs of α-galactosylceramide have been synthesized heretofore, and the correlation between the structures and activities thereof has been investigated. It has been clarified that, in a series of synthetic analogs, KRN7000 developed by Kirin Brewery Company, Limited, which is represented by the formula:
(hereinafter to be referred to as “α-GalCer”) induces the strongest activity, and further, that the corresponding β-configuration (β-GalCer) does not show an immunostimulatory activity (non-patent document 7).
Taking note of such function of NKT cells, therapeutic drugs containing α-GalCer as an active ingredient have been proposed or developed in recent years. However, NKT cells activated by the administration of α-GalCer simultaneously produce, along with the production of IFN-γ, which is a cytokine that induces an immunostimulatory activity and is useful for cancer treatment, IL-4, which is a cytokine that induces an immunosuppressive action. Consequently, problems occur in that the both activities are offset, and the effect for a cancer treatment is not sufficient.
As mentioned above, a glycolipid represented by the formula:
(hereinafter to be referred to as α-C-GalCer) that allows preferential production of IFN-γ, which is a cytokine that induces immunostimulatory activity against NKT cells, has been developed (non-patent documents 8-10, patent documents 1-3). α-C-GalCer is an analog wherein the oxygen atom at the binding site between the sugar and ceramide in α-GalCer is substituted by a methylene group. It has been found that the in vivo stability is enhanced and the efficacy is maintained for a long time since, in α-C-GalCer, the bond between sugar and ceramide is converted from a glycosyl bond to a carbon-carbon bond (non-patent document 11). However, α-C-GalCer is difficult for clinical application, since it leads only a very weak activity on human NKT cells in vitro.
On the other hand, of the present inventors, TASHIRO et al. independently found that a glycolipid having a carbasugar represented by the formula:
strongly induces IFN-γ production by NKT cells (non-patent document 12, patent document 6). In addition, since the glycolipid also induces a strong activity in the human system (in vitro), its clinical application is expected. However, since synthesis of the glycolipid requires many steps, the development of a novel analog has been still desired.
Glycolipids having an amide bond in the ceramide moiety have also been disclosed in patent documents 4 and 5, and non-patent documents 13-16.